An electric power steering apparatus that energizes a steering apparatus of a vehicle by using a rotational torque of a motor as an assist torque, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the assist torque (the steering assist force), such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of a duty ratio of a PWM (Pulse Width Modulation) control.
A general configuration of an electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft) 2 connected to a steering wheel (a handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting the steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit 100 for controlling the electric power steering apparatus from a battery 14, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a current command value of an assist (steering assist) command based on a steering torque T detected by the torque sensor 10 and a velocity V detected by a velocity sensor 12, and controls a current supplied to the motor 20 based on a current control value E obtained by performing compensation and so on with respect to the current command value. Furthermore, it is also possible to receive the velocity V from a CAN (Controller Area Network) and so on.
The control unit 100 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 100 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque T detected by the torque sensor 10 and the velocity V from the velocity sensor 12 are inputted into a current command value calculating section 101, and the current command value calculating section 101 calculates a current command value Iref1 based on the steering torque T and the velocity V. The steering torque T, a motor angular velocityω, and a motor angular acceleration ω* are inputted into a torque compensation section 110, and a torque compensation value Cm is calculated. The current command value Iref1 calculated by the current command value calculating section 101, is phase-compensated by a phase compensation section 102 for enhancing stability of the steering system. A current command value Iref2 phase-compensated by the phase compensation section 102, and the torque compensation value Cm calculated by the torque compensation section 110 are inputted into an adding section 103. The adding section 103 outputs a current command value Iref3 that is the result of addition. The current command value Iref3 is inputted into a maximum current value limiting section 104, and the maximum current value limiting section 104 outputs a current command value Iref4 that the maximum is limited.
The current command value Iref4 is inputted into a subtracting section 105, and the subtracting section 105 obtains a deviation (Iref4−i) of the current command value Iref4 and a motor current value “i” that is fed back. The deviation (Iref4−i) is PI-controlled by a PI control section (a proportional-integral control section) 106, and further is inputted into a PWM control section (a Pulse Width Modulation control section) 107 to perform the adjustment of the duty ratio. The PWM control section 107 outputs the current control value E to an inverter 108, and the inverter 108 controls the motor 20 based on the current control value E. The motor current value “i” of the motor 20 is detected by a motor current detection means 21, and is inputted into the subtracting section 105 to be fed back.
The motor 20 is equipped with a rotation sensor 22 such as a resolver and so on, a motor rotation signal θ from the rotation sensor 22 is inputted into a motor angular velocity calculating section 23, and the motor angular velocity calculating section 23 calculates the motor angular velocity ω that is a rotational angular velocity of the motor 20. Furthermore, the motor angular velocity ω is inputted into a motor angular acceleration calculating section 24, and the motor angular acceleration calculating section 24 calculates the motor angular acceleration ω* that is a rotational angular acceleration of the motor 20 and outputs the calculated motor angular acceleration ω*.
For example, the torque compensation section 110 comprises a differential compensation section 112, a convergence control section 113, an inertia compensation section 114, and so on. The differential compensation section 112 outputs a differential steering torque TA obtained by differentiating the steering torque T to enhance the responsibility, the convergence control section 113 outputs a convergence control value Ga based on the motor angular velocity ω, and the convergence control value Ga and the differential steering torque TA are added in an adding section 116. Moreover, the inertia compensation section 114 outputs an inertia compensation value INa based on the motor angular acceleration ω*, the inertia compensation value INa and an output value of the adding section 116 are added in an adding section 117, and the adding section 117 outputs the torque compensation value Cm to the adding section 103.
The convergence control section 113 applies a brake to a swing operation of the steering wheel in order to improve convergence of the vehicle yaw. The inertia compensation section 114 removes a torque that accelerates or decelerates the motor inertia from the steering torque T, and generates a steering feeling without an inertia feeling.
Here, for solving problems about assembling the steering mechanism and for purposes such as absorbing displacements in an axis direction and vibrations that occur during vehicle running, an intermediate shaft mechanism in which an intermediate shaft comprised of an expansion/contraction shaft is arranged in the middle part of the column shaft 2 of the steering mechanism, is used recently. FIG. 3 shows the appearance of the steering mechanism comprising such an intermediate shaft 4 as corresponding to FIG. 1. As shown in FIG. 3, the motor 20 is installed in a drive mechanism unit 30 that comprises the torque sensor 10, the reduction gears 3 and so on, and the expandable and contractive intermediate shaft 4 is arranged between universal joints 4a and 4b of the middle part of the column shaft 2.
For example, the details of the intermediate shaft 4 are a structure shown in FIG. 4. That is to say, the intermediate shaft 4 comprises an outer tube 41 and an inner shaft 42, the outer tube 41 has a yoke 4b-1 which is welded to an end portion and forms the universal joint 4b, and the inner shaft 42 has a yoke 4a-1 which is welded to an end portion and forms the universal joint 4a. A female spline 43 is formed on an inner peripheral surface of the outer tube 41, on the other hand, a male spline 45 interdigitated with the female spline 43 is formed on an outer peripheral surface of a tip portion 44 of the inner shaft 42. Additionally, main constructional members of the universal joint 4b are the yoke 4b-1, a joint yoke 4b-2 and a spider 4b-3. Further, at least one surface of the female spline 43 and the male spline 45 is coated by a low-friction resin such as a PTFE (polytetrafluoroethylene) or a polyamide resin.
The above-described general electric power steering apparatus is equipped with a rack end mechanism for stopping steering of the steering wheel above a certain level, by steering the steering wheel from the neutral position to right and left given rack end angles respectively, when the steering angle of the steering wheel reaches a maximum steering angle, it becomes impossible to steer the steering wheel in the same direction to an angle more than the maximum steering angle. For this reason, despite the steering wheel is steered to the neighborhood of the rack end angle, when responding to that a large steering torque is applied to the steering wheel and a large steering assist force is applied to the steering apparatus from the motor, there is a possibility that a large shock is applied to the steering mechanism, loud shock noises occur, and damages and deformations of component parts of the steering mechanism occur.
As an apparatus to solve such problems, for example, there has been an apparatus disclosed in Japanese Examined Patent Application Publication No. H6-4417 B2 (Patent Document 1). In the apparatus described in Patent Document 1, after the steering angle of the steering wheel reaches a given angle located to adjacent to the rack end angle, by decreasing a desired current value with an increase in the steering angle and setting a desired current value to zero when the steering angle reaches the rack end angle, it becomes possible to prevent a situation that a large shock is applied to the steering mechanism. That is to say, as indicated in a dashed line of FIG. 5, within a range AR from the given angle to the maximum steering angle that is the steering end, an armature current Ia of the motor gradually decreases, and the armature current Ia is controlled so as to become zero, furthermore, a steering torque Lp that is an assist torque applied by the motor gradually decreases with an increase in a load torque, and becomes equal to a steering torque Tm during a manual operation in the maximum steering angle of the steering wheel, so the motor is not driven in the stroke end portion of the steering wheel, it is possible to prevent occurrences of an overload status of the motor and a heat generation within the motor, and simultaneously it is possible to reduce the power consumption. That is, the apparatus described in Patent Document 1, decreases the desired current value in the case that the steering angle is equal to or more than the given value, and aims at improving durability of the steering mechanism such as the intermediate shaft, the tie rods, the rack and pinion mechanism and the hub units and reducing the power consumption of the motor.
However, in the apparatus described in the above Patent Document 1, a mechanism that decreases the assist torque is consistently constant, no consideration is given to a thing that durability of the steering mechanism diminishes on a long-term basis, therefore the need of an improvement in permanent durability is requested.